The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are a seed layer (not shown) on which is an antiferromagnetic layer 11 whose purpose is to act as a pinning agent for magnetically pinned layer 12. Next is a copper spacer layer 13 on which is low coercivity (free) ferromagnetic layer 14. When free layer 14 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will be at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase.
Also, as seen in FIG. 1, the GMR stack may be given increased stability by the provision of a second pinning/pinned pair (18 and 17 respectively) that are separated from free layer 14 by decoupling layer 15. The action of the latter is similar to that of spacer 13 except that layer 15 affects the bias while layer 13 affects the GMR ratio.
The example seen in FIG. 1 is a GMR device that measures the resistance of the free layer for current flowing parallel to its two surfaces (CIP). A standard feature of such devices are permanent magnets 19 that but up against the free layer in order to provide longitudinal magnetic bias (and hence stability) at the ends of the free layer. As can be seen, because of the taper in the stack profile, the thickness of the bias layer is somewhat greater at the bottom of the free layer than at its top, so the bias layer partly overlies the free layer, thereby limiting its effectiveness to some extent.
As the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. An example of such a device is shown in FIG. 2. The main differences from the CIP device are the top and bottom conductor layers 21 and 22 which ensure that the sensing current of the device passes in a direction normal to the free and spacer layers.
A related device to the CPP GMR described above is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. An MTJ is readily visualized by substituting a dielectric layer for spacer layer 16 in FIG. 2. The principle governing the operation of the MTJ in magnetic read sensors is the change of resistivity of the tunnel junction between two ferromagnetic layers when it is subjected to a bit field from magnetic media. When the magnetizations of the pinned and free layers are in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically 40%, which is much larger than for GMR devices.
A routine search of the prior art was performed with the following references of interest being found:
Haase et al. note that Xe ion beam etching is known (U.S. Pat. No. 6,058,123). In U.S. Pat. No. 6,554,974, Shiratori confirms that any of several heavy ions, including Xe ions, may be used for sputter etching but does not disclose any particular advantage of xenon over the others. The invention utilizes the known fact that sputter rate increases with angle of incidence to preferentially remove material from the sloping sides of write tracks within a recording surface. U.S. Pat. No. 6,002,553 (Stearns et al.) and U.S. Pat. No. 6,421,212 (Gibbons et al) teach ion beam etching to form CPP GMR sensors.